That's an interesting question, and the curiosity of it relies on the fact that hand has multiple meanings.
Single-handed has two meanings, and each gives a different answer to the question:
Note that in the first sense of working without others to help uses the sense of hand not as a body part but as a worker, which is commonly seen in ships: All hands on deck.
I am providing a link to a interesting page that lists many phrases using the word hand in different ways.
Yes if they do it one at a time ... lol
A group of people working on something.
Absolutely. An antagonist can be any single person or group of people that opposes the protagonist(s).
Quartet.
Nation
As a group, it is a population; a single person is a citizen. The people of a place are also called inhabitants; or a single person is an inhabitant.
a single person or group
a single person or a small group of people
propaganda
The number of people who aren't in a given area.
A group of people doing something together.
Committee
consensus